This articleis missing information about a succinct statement of the theorem. Please expand the article to include this information. Further details may exist on thetalk page.(September 2024)
The principles of Mendelian inheritance were named for and first derived byGregor Johann Mendel,[3] a nineteenth-centuryMoravianmonk who formulated his ideas after conducting simple hybridization experiments with pea plants(Pisum sativum) he had planted in the garden of his monastery.[4] Between 1856 and 1863, Mendel cultivated and tested some 5,000 pea plants. From these experiments, he induced two generalizations which later became known asMendel's Principles of Heredity orMendelian inheritance. He described his experiments in a two-part paper,Versuche über Pflanzen-Hybriden (Experiments on Plant Hybridization),[5] that he presented to the Natural History Society ofBrno on 8 February and 8 March 1865, and which was published in 1866.[3][6][7][8]
Mendel's results were at first largely ignored. Although they were not completely unknown to biologists of the time, they were not seen as generally applicable, even by Mendel himself, who thought they only applied to certain categories of species or traits. A major roadblock to understanding their significance was the importance attached by 19th-century biologists to theapparent blending ofmany inherited traits in the overall appearance of the progeny,[citation needed] now known to be due tomulti-gene interactions, in contrast to the organ-specific binary characters studied by Mendel.[4] In 1900, however, his work was "re-discovered" by three European scientists,Hugo de Vries,Carl Correns, andErich von Tschermak. The exact nature of the "re-discovery" has been debated: De Vries published first on the subject, mentioning Mendel in a footnote, while Correns pointed out Mendel's priority after having read De Vries' paper and realizing that he himself did not have priority. De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work and how much came only after reading Mendel's paper. Later scholars have accused Von Tschermak of not truly understanding the results at all.[9][10]
Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe wasWilliam Bateson, who coined the terms "genetics" and "allele" to describe many of its tenets.[11] The model ofheredity was contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable for many traits.[12] Many biologists also dismissed the theory because they were not sure it would apply to all species. However, later work by biologists and statisticians such asRonald Fisher showed that if multiple Mendelian factors were involved in the expression of an individual trait, they could produce the diverse results observed, thus demonstrating that Mendelian genetics is compatible withnatural selection.[13][14]Thomas Hunt Morgan and his assistants later integrated Mendel's theoretical model with thechromosome theory of inheritance, in which the chromosomes ofcells were thought to hold the actual hereditary material, and created what is now known asclassical genetics, a highly successful foundation which eventually cemented Mendel's place in history.[3][11]
Mendel's findings allowed scientists such as Fisher andJ.B.S. Haldane to predict the expression of traits on the basis of mathematical probabilities. An important aspect of Mendel's success can be traced to his decision to start his crosses only with plants he demonstrated weretrue-breeding.[4][13] He only measured discrete (binary) characteristics, such as color, shape, and position of the seeds, rather than quantitatively variable characteristics. He expressed his results numerically and subjected them tostatistical analysis. His method of data analysis and his largesample size gave credibility to his data. He had the foresight to follow several successive generations (P, F1, F2, F3) of pea plants and record their variations. Finally, he performed "test crosses" (backcrossing descendants of the initialhybridization to the initial true-breeding lines) to reveal the presence and proportions ofrecessive characters.[15]
Punnett squares are a well known genetics tool that was created by an English geneticist,Reginald Punnett, which can visually demonstrate all the possible genotypes that an offspring can receive, given the genotypes of their parents.[16][17][18] Each parent carries two alleles, which can be shown on the top and the side of the chart, and each contribute one of them towards reproduction at a time. Each of the squares in the middle demonstrates the number of times each pairing of parental alleles could combine to make potential offspring. Using probabilities, one can then determine which genotypes the parents can create, and at what frequencies they can be created.[16][18]
For example, if two parents both have a heterozygous genotype, then there would be a 50% chance for their offspring to have the same genotype, and a 50% chance they would have a homozygous genotype. Since they could possibly contribute two identical alleles, the 50% would be halved to 25% to account for each type of homozygote, whether this was a homozygous dominant genotype, or a homozygous recessive genotype.[16][17][18]
Pedigrees are visual tree like representations that demonstrate exactly how alleles are being passed from past generations to future ones.[19] They also provide a diagram displaying each individual that carries a desired allele, and exactly which side of inheritance it was received from, whether it was from their mother's side or their father's side.[19] Pedigrees can also be used to aid researchers in determining the inheritance pattern for the desired allele, because they share information such as the gender of all individuals, the phenotype, a predicted genotype, the potential sources for the alleles, and also based its history, how it could continue to spread in the future generations to come. By using pedigrees, scientists have been able to find ways to control the flow of alleles over time, so that alleles that act problematic can be resolved upon discovery.[20]
Five parts of Mendel's discoveries were an important divergence from the common theories at the time and were the prerequisite for the establishment of his rules.
Characters are unitary, that is, they are discrete e.g.: purplevs. white, tallvs. dwarf. There is no medium-sized plant or light purple flower.
Genetic characteristics have alternate forms, each inherited from one of two parents. Today these are calledalleles.
One allele is dominant over the other. The phenotype reflects the dominant allele.
Gametes are created by random segregation. Heterozygotic individuals produce gametes with an equal frequency of the two alleles.
Different traits have independent assortment. In modern terms, genes are unlinked.
According to customary terminology, the principles of inheritance discovered by Gregor Mendel are here referred to as Mendelian laws, although today's geneticists also speak ofMendelian rules orMendelian principles,[21][22] as there are many exceptions summarized under the collective termNon-Mendelian inheritance. The laws were initially formulated by the geneticistThomas Hunt Morgan in 1916.[23]
Characteristics Mendel used in his experiments[24]P-Generation and F1-Generation: The dominant allele for purple-red flower hides the phenotypic effect of the recessive allele for white flowers. F2-Generation: The recessive trait from the P-Generation phenotypically reappears in the individuals that are homozygous with the recessive genetic trait.Myosotis: Colour and distribution of colours are inherited independently.[25]
Mendel selected for the experiment the following characters of pea plants:
Form of the ripe seeds (round or roundish, surface shallow or wrinkled)
Colour of theseed–coat (white, gray, or brown, with or without violet spotting)
When he crossed purebred white flower and purple flower pea plants (the parental or P generation) byartificial pollination, the resulting flower colour was not a blend. Rather than being a mix of the two, the offspring in the first generation (F1-generation) were all purple-flowered. Therefore, he called thisbiological trait dominant. When he allowedself-fertilization in the uniform looking F1-generation, he obtained both colours in the F2 generation with a purple flower to white flower ratio of 3 : 1. In some of the other characters also one of the traits was dominant.
He then conceived the idea of heredity units, which he called hereditary "factors". Mendel found that there are alternative forms of factors—now calledgenes—that account for variations in inherited characteristics. For example, the gene for flower color in pea plants exists in two forms, one for purple and the other for white. The alternative "forms" are now calledalleles. For each trait, an organism inherits two alleles, one from each parent. These alleles may be the same or different. An organism that has two identical alleles for a gene is said to behomozygous for that gene (and is called a homozygote). An organism that has two different alleles for a gene is said to beheterozygous for that gene (and is called a heterozygote).
Mendel hypothesized that allele pairs separate randomly, or segregate, from each other during the production of thegametes in the seed plant (egg cell) and the pollen plant (sperm). Because allele pairs separate during gamete production, asperm oregg carries only one allele for each inherited trait. When sperm and egg unite atfertilization, each contributes its allele, restoring the paired condition in the offspring. Mendel also found that each pair of alleles segregates independently of the other pairs of alleles during gamete formation.
Thegenotype of an individual is made up of the many alleles it possesses. Thephenotype is the result of theexpression of all characteristics that are genetically determined by its alleles as well as by its environment. The presence of an allele does not mean that the trait will be expressed in the individual that possesses it. If the two alleles of an inherited pair differ (the heterozygous condition), then one determines the organism's appearance and is called thedominant allele; the other has no noticeable effect on the organism's appearance and is called therecessive allele.
Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.[27]
Law of segregation
During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
Law of independent assortment
Genes of different traits can segregate independently during the formation of gametes.
F1 generation: All individuals have the same genotype and same phenotype expressing the dominant trait (red). F2 generation: The phenotypes in the second generation show a 3 : 1 ratio. In the genotype 25 % are homozygous with the dominant trait, 50 % are heterozygousgenetic carriers of the recessive trait, 25 % are homozygous with the recessive genetic trait andexpressing the recessive character.InMirabilis jalapa andAntirrhinum majus are examples for intermediate inheritance.[28][29] As seen in the F1-generation, heterozygous plants have "light pink" flowers—a mix of "red" and "white". The F2-generation shows a 1:2:1 ratio ofred:light pink:white.
If two parents are mated with each other who differ in onegenetic characteristic for which they are bothhomozygous (each pure-bred), all offspring in the first generation (F1) are equal to the examined characteristic ingenotype andphenotype showing the dominant trait. Thisuniformity rule orreciprocity rule applies to all individuals of the F1-generation.[30]
The principle of dominant inheritance discovered by Mendel states that in a heterozygote the dominant allele will cause the recessive allele to be "masked": that is, not expressed in the phenotype. Only if an individual is homozygous with respect to the recessive allele will the recessive trait be expressed. Therefore, a cross between a homozygous dominant and a homozygous recessive organism yields a heterozygous organism whose phenotype displays only the dominant trait.
The F1 offspring of Mendel's pea crosses always looked like one of the two parental varieties. In this situation of "complete dominance", the dominant allele had the same phenotypic effect whether present in one or two copies.
But for some characteristics, the F1 hybrids have an appearancein between the phenotypes of the two parental varieties. A cross between two four o'clock (Mirabilis jalapa) plants shows an exception to Mendel's principle, calledincomplete dominance. Flowers of heterozygous plants have a phenotype somewhere between the two homozygous genotypes. In cases of intermediate inheritance (incomplete dominance) in the F1-generation Mendel's principle of uniformity in genotype and phenotype applies as well. Research about intermediate inheritance was done by other scientists. The first wasCarl Correns with his studies about Mirabilis jalapa.[28][31][32][33][34]
The law of segregation of genes applies when two individuals, both heterozygous for a certain trait are crossed, for example, hybrids of the F1-generation. The offspring in the F2-generation differ in genotype and phenotype so that the characteristics of the grandparents (P-generation) regularly occur again. In a dominant-recessive inheritance, an average of 25% are homozygous with the dominant trait, 50% are heterozygous showing the dominant trait in the phenotype (genetic carriers), 25% are homozygous with the recessive trait and thereforeexpress the recessive trait in the phenotype. The genotypic ratio is 1: 2 : 1, and the phenotypic ratio is 3: 1.
In the pea plant example, the capital "B" represents the dominant allele for purple blossom and lowercase "b" represents the recessive allele for white blossom. Thepistil plant and thepollen plant are both F1-hybrids with genotype "B b". Each has one allele for purple and one allele for white. In the offspring, in the F2-plants in the Punnett-square, three combinations are possible. The genotypic ratio is 1BB : 2Bb : 1bb. But the phenotypic ratio of plants with purple blossoms to those with white blossoms is 3 : 1 due to the dominance of the allele for purple. Plants with homozygous "b b" are white flowered like one of the grandparents in the P-generation.
In cases ofincomplete dominance the same segregation of alleles takes place in the F2-generation, but here also the phenotypes show a ratio of 1 : 2 : 1, as the heterozygous are different in phenotype from the homozygous because thegenetic expression of one allele compensates the missing expression of the other allele only partially. This results in an intermediate inheritance which was later described by other scientists.
In some literature sources, the principle of segregation is cited as the "first law". Nevertheless, Mendel did his crossing experiments with heterozygous plants after obtaining these hybrids by crossing two purebred plants, discovering the principle of dominance and uniformity first.[35][27]
Molecular proof of segregation of genes was subsequently found through observation ofmeiosis by two scientists independently, the German botanistOscar Hertwig in 1876, and the Belgian zoologistEdouard Van Beneden in 1883. Most alleles are located inchromosomes in thecell nucleus. Paternal and maternal chromosomes get separated in meiosis because duringspermatogenesis the chromosomes are segregated on the four sperm cells that arise from one mother sperm cell, and duringoogenesis the chromosomes are distributed between thepolar bodies and theegg cell. Every individual organism contains two alleles for each trait. They segregate (separate) during meiosis such that eachgamete contains only one of the alleles.[36] When the gametes unite in thezygote the alleles—one from the mother one from the father—get passed on to the offspring. An offspring thus receives a pair of alleles for a trait by inheritinghomologous chromosomes from the parent organisms: one allele for each trait from each parent.[36] Heterozygous individuals with the dominant trait in the phenotype aregenetic carriers of the recessive trait.
Segregation and independent assortment are consistent with thechromosome theory of inheritance.Precondition for the example: Two parent dogs (P-generation) are homozygous for two different genetic traits. In each case one parent has the dominant, one the recessive allele. Their offsprings in the F1-generation are heterozygous at both loci and show the dominant traits in their phenotypes according to the law of dominance and uniformity. Now two heterozygous mature individuals of such F1-generation are bred together. The dominant allele "E" (on the extension locus) provides black eumelanin in the coat. The recessive allele "e" (on the extension locus) hinders the storage of eumelanin in the coat, so only the pigments for the "Tan" colour are in the coat. The dominant allele S (on the S-locus) provides for the pigmentation of the entire coat. The recessive allele sP (on the S-locus) causes a whitePiebald spotting.[37] Now in the puppies in theF2-generation all combinations are possible. The Piebald spotting and the genes for the different colour pigments are inherited independently of each other.[38] Average number ratio of phenotypes 9:3:3:1.[39]For example 3 pairs of homologous chromosomes allow 8 possible combinations, all equally likely to move into the gamete duringmeiosis. This is the main reason for independent assortment. The equation to determine the number of possible combinations given the number of homologous pairs = 2x (x = number of homologous pairs)
The law of independent assortment proposes alleles for separate traits are passed independently of one another.[40][35] That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait. Mendel found support for this law in his dihybrid cross experiments. In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios. This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each.
Independent assortment occurs ineukaryotic organisms during meiotic metaphase I, and produces a gamete with a mixture of the organism's chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes. Along withcrossing over, independent assortment increases genetic diversity by producing novel genetic combinations.
There are many deviations from the principle of independent assortment due togenetic linkage.
Of the 46 chromosomes in a normaldiploid human cell, half are maternally derived (from the mother'segg) and half are paternally derived (from the father'ssperm). This occurs assexual reproduction involves the fusion of twohaploid gametes (the egg and sperm) to produce a zygote and a new organism, in which every cell has two sets of chromosomes (diploid). Duringgametogenesis the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another haploid gamete to produce a diploid organism.
In independent assortment, the chromosomes that result are randomly sorted from all possible maternal and paternal chromosomes. Because zygotes end up with a mix instead of a pre-defined "set" from either parent, chromosomes are therefore considered assorted independently. As such, the zygote can end up with any combination of paternal or maternal chromosomes. For human gametes, with 23 chromosomes, the number of possibilities is 223 or 8,388,608 possible combinations.[41] This contributes to the genetic variability of progeny. Generally, the recombination of genes has important implications for many evolutionary processes.[42][43][44]
A Mendelian trait is one whose inheritance follows Mendel's principles—namely, the trait depends only on a singlelocus, whosealleles are eitherdominant or recessive.
Mendel himself warned that care was needed in extrapolating his patterns to other organisms or traits. Indeed, many organisms have traits whose inheritance works differently from the principles he described; these traits are called non-Mendelian.[46][47]
For example, Mendel focused on traits whose genes have only two alleles, such as "A" and "a". However, many genes havemore than two alleles. He also focused on traits determined by a single gene. But some traits, such as height, depend on many genes rather than just one. Traits dependent on multiple genes are calledpolygenic traits.
^Simunek, Michal V. (January 2011).The Mendelian Dioskuri. Correspondence of Armin with Erich von Tschermak-Seysenegg, 1898-1951. Pavel Mervart & Institute of Contemporary History of the AcSc Prague.ISBN978-80-87378-67-0.
^abcEdwards, A. W. F. (1 March 2012)."Punnett's square".Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences. Data-Driven Research in the Biological and Biomedical Sciences.43 (1):219–224.doi:10.1016/j.shpsc.2011.11.011.ISSN1369-8486.PMID22326091.
Bowler, Peter J. (1989).The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society. Johns Hopkins University Press.
Atics, Jean.Genetics: The life of DNA. ANDRNA press.
Reece, Jane B.; Campbell, Neil A. (2011).Mendel and the Gene Idea (9th ed.). Benjamin Cummings / Pearson Education. p. 265.
